This invention relates generally to a method for compression molding. Specifically, the present invention relates to molding to an object called an insert and to entrapping the molding compound in a cavity formed between the insert and a single mold plate.
Three techniques of conventional molding are known: compression, transfer, and injection. Each technique differs from the others in the type of molding compound typically used and the approach taken to load the compound into a mold cavity. Thermosettting resins, which become permanently rigid when heated, are typically used in compression and transfer molding. Compression molding requires individual handling of the compound. However, in transfer molding the compound is automatically forced into one or more mold cavities by an independent part of the mold structure. On the other hand, injection molding typically uses a thermoplastic compound, which softens when heated and hardens when cooled. Furthermore, injection molding uses an external structure to automatically force the compound into a tightly closed mold that may contain several cavities connected together by a series of branch passages.
Compression molding offers certain advantageous over the other molding techniques. The tooling costs are lower because no mechanism is required to automatically load molding compound. Further, the molding compound escapes being masticated by automatic loading mechanisms. Thus, the molded objects demonstrate higher mechanical properties. This is especially significant when the molding compound contains internal fibers for strenghtening the molded object. Such fibers tend to remain randomly oriented and unbroken, and thus produce stronger molded objects having tighter ID and OD tolerances.
The existing compression molding methods require the use of a mold comprised of at least two complimentary mold plates constructed so that the plates are mated together a mold cavity remains between the plates. First, the mold is heated to a predetermined temperature, and a predetermined quantity of a thermosetting resin is measured. Then, the mold is opened and the measured quantity of thermosetting resin is placed in the mold cavity between the mold plates. Next, the mold plates are clamped together and remain clamped for a predetermined period of time. During this period the heat in the mold and the clamping pressure transfer to the thermosetting resin causing the resin to assume the shape of the cavity and to undergo an irreversible chemical reaction known as curing. Next, the mold plates are momentarily separated to allow curing gasses to escape from them mold cavity, a step known as breathing, and then the plates are clamped together again for another predetermined period of time. Finally, the mold plates are again separated and the resulting molded object is removed typically by using ejector pins. The use of ejector pins poses a problem because they tend to make scarring marks on the molded object. Flash, which forms during curing when excess resin escapes the cavity at the parting line between the mold plates, poses an additional problem because it must then be removed from the finished object.
The existing compression molding methods contain several disadvantages which are improved in the present invention. Specifically, the multiple mold plates, the breathing step, the use of ejector pins, and the formation of flash are all seen as causing an unnecessary complication, expense, and waste of time in a method for compression molding, and are thus addressed in the present invention.